1. Field of the Invention
The present invention relates to earth-boring bits of the rolling cutter variety. Specifically, the present invention relates to the cutting structure and cutting elements of earth-boring bits of the rolling cutter variety.
2. Background Information
The success of rotary drilling enabled the discovery of deep oil and gas reserves. The rotary rock bit was an important invention that made that success possible. Only soft formations could be commercially penetrated with the earlier drag bit, but the original rolling-cone rock bit invented by Howard R. Hughes, U.S. Pat. No. 939,759, drilled the hard caprock at the Spindletop field, near Beaumont Texas, with relative ease.
That venerable invention, within the first decade of this century, could drill a scant fraction of the depth and speed of the modern rotary rock bit. If the original Hughes bit drilled for hours, the modern bit drills for days. Bits today often drill for miles. Many individual improvements have contributed to the impressive overall improvement in the performance of rock bits.
Rolling-cutter earth-boring bits generally employ cutting elements to induce high contact stresses in the formation being drilled as the cutters roll over the bottom of the borehole during drilling operation. These stresses cause the rock to fail, resulting in disintegration through near-vertical penetration of the formation material being drilled. When cutters are offset, their axes do not coincide with the geometric or rotational axis of the bit and a small component of horizontal or sliding motion is imparted to the cutters as they roll over the borehole bottom. While this drilling mode prevails on the borehole bottom, it is entirely different in the corner and on the sidewall. The corner is generated by a combined crushing and scraping or shearing action, while the borehole wall is produced in a pure sliding and scraping (shearing) mode. In the corner and on the sidewall of the borehole, the cutting elements have to do the most work and are subjected to extreme stresses, which makes them prone to break down prematurely, and/or wear rapidly.
Recently, there has been a general effort to introduce the improved material properties of natural and synthetic diamond or super-hard materials into earth-boring bits of the rolling-cutter variety. Super-hard materials have been used in fixed-cutter or drag bits to good effect for many years. Fixed-cutter bits employ the shearing mode of disintegration discussed above almost exclusively. Although diamond and other super-hard materials possess excellent hardness and other material properties, they generally are considered too brittle for most cutting element applications in rolling-cutter bits, an exception being the shear-cutting gage inserts discussed above.
Recent attempts to introduce diamond and similar materials into rolling cutter bits have relied on a diamond layer or table secured to a substrate or backing material of fracture-tough hard metal, usually cemented tungsten carbide. The substrate is thought to supplement the diamond or super-hard material with its increased toughness, resulting in a cutting element with satisfactory hardness and toughness, which diamond alone is not thought to provide.
One problem with the diamond/substrate inserts is the tendency of the diamond or super-hard material to delaminate from the substrate. The cause of this delamination is thought to be forces acting parallel to the interface between the diamond layer or table and the substrate superimposed on the high residual stresses at this interface. These stresses shear the diamond table off of its substrate.
Several attempts have been made to increase the strength of the interface. U.S. Pat. No. 4,604,106, to Hall et al. discloses a transition layer interface that gradually transitions between the properties of the super-hard material and the substrate material at the interface between them to resist delamination. Although this method appears to yield satisfactory results, it requires expensive and time-consuming fabrication techniques. Other patents, such as commonly assigned U.S. Pat. No. 5,351,772, Oct. 4, 1994 to Smith, provide a non-planar interface between the diamond table and substrate. U.S. Pat. No. 5,355,969 to Hardy et al. is another example of the non-planar interface between the super-hard and substrate.
At any rate, most attempts to incorporate diamond or other super-hard materials into the cutting structures of earth-boring bits of the rolling-cutter variety employ a non-diamond substrate material in addition to the super-hard material.
A need exists, therefore, for earth-boring bits of the rolling-cutter variety having super-hard cutting elements that are relatively easily manufactured with a satisfactory combination of material properties.